1. Field of the Invention
The present invention relates to a light source device employing a laser diode and used as a writing light source in an image forming apparatus, for example, as exemplified by a printer, a facsimile machine or a copier, and an optical scanning apparatus provided with such a light source device.
2. Description of the Related Art
There has been conventionally used an optical scanning apparatus for illuminating a photoconductive element with a laser beam outputted from a laser emitting portion to form an electrostatic latent image. Generally, the optical scanning apparatus is constructed such that a beam light emitted from a beam light source device having a laser diode (hereinafter, referred to as an “LD”) is focused as a beam spot on a surface to be scanned by way of scanning optical members including a cylindrical lens, a polygon mirror, and a scanning lens. Scanning of the beam spot at the same speed on the surface to be scanned in a main scanning direction is realized through the rotation of the polygon mirror. A multi-beam scanning apparatus provided with a plurality of LDs can more quickly scan a surface and form an electrostatic latent image without increasing the number of revolutions of the polygon mirror as compared to a case where scanning is performed using one beam light. Thus, such a multi-beam scanning apparatus is widely applicable to an image forming apparatus as exemplified by a copier, a laser printer or a laser facsimile machine.
In such an optical scanning apparatus, an interval adjustment of beam spots formed on a surface to be scanned largely depends on a precision adjustment of an optical-axis alignment of a light source device, the precision of a beam splitter surface and the angular precision of a reflecting surface. Depending on the material and shape thereof or a difference in thermal expansion coefficient between the base member and a housing of the optical scanning apparatus, however, a base member on which the light source device is to be mounted and an adjustment member for beam spots are deformed by heat produced from the LDs themselves and an APC (automatic power control) board for performing a modulation control of the LDs, or when an ambient temperature largely changes. As these members are deformed, an installed angle of the base member varies from a desired angle, thereby causing a problem of displacing an array of beam spots in a sub-scanning direction.
In view of this problem, various methods have been proposed to prevent an array displacement of beam spots in the sub-scanning direction. Japanese Unexamined Patent Publication No. 2002-162591 discloses a method according to which an LD control plate for performing a modulation control of an LD in accordance with an image data is exposed to the outside of an optical scanning apparatus, thereby preventing a thermal deformation of a base member caused by heat from the LD control plate. However, according to this embodiment, an exposure space for the LD control plate is necessary because the LD control plate is exposed to the outside of the apparatus to radiate heat outside, which is disadvantageous to the miniaturization and higher density of an image forming apparatus. Further, since it is necessary to seal the apparatus by employing a labyrinth structure or the like, the construction of the housing becomes complicated. In the case of a multi-beam optical scanning apparatus using a plurality of LDs, process precision and mass productivity are reduced, leading to a cost increase.
Japanese Unexamined Patent Publication No. 2003-29180 discloses a method according to which a restricting means for restricting an angle of inclination of a light source unit with respect to a sub-scanning direction is movably provided so that a change in the inclination of the light source unit resulting from a temperature change can be adjusted. This method necessitates the restricting member in addition to a base member, thereby increasing the number of parts and increasing adjustment steps because of a more complicated construction. Further, a nonlinear element is added to make the construction instable since an elastic member is disposed between the base member and the restricting member.
Further, Japanese Unexamined Patent Publication No. 2002-350760 discloses a method according to which, in an optical scanning apparatus including a first optical system for coupling luminous fluxes from a light source, a second optical system for focusing the coupled luminous fluxes as a line image on a plane of polarization and a third optical system for gathering the focused beam light as a spot on a surface to be scanned, temperature-distribution creating means is provided to make a change in ambient temperature near the third optical system smaller than those near the first and second optical systems. According to this embodiment, the temperature-distribution creating means such as an air-cooling fan needs to be separately provided, which causes the enlargement and higher cost of the optical scanning apparatus.